1. Field of the Invention
The present invention relates generally to a wireless cellular communication system, more particularly, to an Orthogonal Frequency Division Multiple Access (OFDMA) wireless communication system, where one or more Orthogonal Frequency Division Multiplexing (OFDM) symbols are allocated as a control channel to convey control information.
2. Description of the Related Art
The control channel is designed to carry the control signaling including Download Control Information (DCI) for purposes of downlink and uplink resource allocation. The design of the control channel may vary from system to system. In 3rd Generation Partnership Project Long Term Evolution (3GPP LTE) Release 8, the Physical Downlink Control CHannel (PDCCH) consists of the first several OFDM symbols of each sub-frame.
In 3GPP LTE Release 8, a Packet Data Control CHannel (PDCCH) is presented in the first several OFDM symbols. The number of OFDM symbols used for PDCCH is indicated in another physical control format indication channel (PCFICH) in the first OFDM symbol. Each PDCCH consists of L Control Channel Elements (CCE), where L=1, 2, 4, 8 representing different CCE aggregation levels; each CCE consists of 9 Resource Element Group (REG) spreading on the transmission bandwidth. Each REG consists of 4 consecutive resource elements exclusive of common reference resource elements. The PDCCH structure is illustrated in FIG. 1.
Upon receiving the PDCCH, the user should try blindly decoding PDCCH on its search space. Search space contains multiple possible CCE starting index and CCE aggregation level. The user try to decode expected DCI format with the assumption, if the CRC check passes, the user will assume a DCI format is successfully received.
In 3GPP LTE release 8 to 10, the search space is designed by the following Equation (1):L={(Yk+m)mod └NCCE,k/L┘}+i   (1)
where,
i=0, . . . , L−1,
m=0, . . . , M(L)−1,
Yk=(A·Yk-1)mod D, Y−1=nRNTI≠0, A=39827, D=65537, and M(L) is defined in the following Table 1:
TABLE 1PDCCH candidates monitored by a User Equipment (UE)Search space Sk(L)Number of PDCCHTypeAggregation level LSize [in CCEs]candidates M(L)UE-166specific21264828162Common41648162
In the legacy 3GPP systems the PDCCH is transmitted using transmit diversity when multiple antennas are available. The user uses the common reference signals inside the PDCCH region for decoding. The common reference signals are cell-specific.
However, by using the cell-specific common reference signal, a user has to decode every DCI format until CRC check passes. As such, the process for decoding can be considered very inefficient.